The present invention relates generally to a method of separating, in units of a chip, semiconductor devices formed on a wafer, and more particularly to a method of separating semiconductor devices, which is suitable for cases where single-crystal substrates with low cleavage such as sapphire substrates are used.
The present invention also relates to a method of separating semiconductor devices, including a step of polishing semiconductor devices wherein Group III nitride semiconductor layers are stacked on a substrate of sapphire, etc.
Recently, attention has been paid to gallium nitride-based compound semiconductors, such as GaN, as materials of short-wavelength light emitting diodes (LED) or semiconductor laser devices (LD) for use in a range between blue light and ultraviolet. The LED using such a material is indispensable in constructing a display as a light emitting device for emitting blue light, i.e., one of the three primary colors of light. It is expected that the blue light semiconductor laser device will be applied to a light source for high-density information processing because of its short oscillation wavelength.
Conventionally gallium nitride-based compound semiconductor layers are grown on a sapphire substrate. In this case, there is a problem in that device separation (i.e., division of devices in units of a chip) is difficult due to low cleavage of sapphire. Some solutions for this problem have been proposed. For example, Jpn. Pat. Appln. KOKAI Publication No. 5-315646 discloses that grooves are formed in a wafer, in which crystals have been grown on a substrate, by means of dicing from the crystal side, and then the substrate is polished, following which scribe lines are formed along the grooves to separate devices.
Applied methods of this technique are described in Jpn. Pat. Appln. KOKAI Publication No. 5-343742, Jpn. Pat. Appln. KOKAI Publication No. 7-273069, Jpn. Pat. Appln. KOKAI Publication No. 8-222807, etc. In these methods, grooves are made in both top and bottom surfaces of wafers and thus devices are separated. Another method is described in Jpn. Pat. Appln. KOKAI Publication No. 6-283758. In this method, after a sapphire substrate is polished, a warped wafer including the sapphire substrate is held and flattened and in this state scribe lines are formed in the wafer and the devices are separated by making use of a restoring force of the wafer.
In the above methods, however, a stress of the polished wafer is inevitably released when it is removed from a polishing jig. At the time of removing the wafer, it may be accidentally divided in an undesirable direction.
When a hard material such as sapphire or SiC is used in forming a substrate, a pressure force, greater than necessary, acts on the substrate. As a result, a strain greater than necessary occurs in the device and the performance of the device deteriorates. According to experiments by the inventors, in particular, when the bottom of the substrate is not flattened, this strain increases and becomes locally non-uniform. Consequently, the performance of the device considerably deteriorates.
As has been described above, in the prior art, when the wafer includes a sapphire substrate as an underlying layer for a semiconductor device, it is necessary to sufficiently polish the sapphire substrate before forming scribe lines to separate devices, because sapphire has low cleavage. In this case, when the polished wafer is removed from the polishing jig, stress of the wafer will be released and the wafer may be divided in an undesirable direction.
Moreover, the crystalline substrate of, e.g., sapphire or SiC, which is used as the underlying layer for the semiconductor device, is very hard. It is very difficult to polish and flatten the crystalline substrate, and as a result the performance of the device will decrease.